The electric vehicle (including a hybrid vehicle and a fuel cell vehicle) obtaining all or a part of the vehicle driving force by an electric motor is equipped with a secondary battery, and the electric motor is driven by the electric power stored in this secondary battery. The function specific to such an electric vehicle includes regenerative braking. During braking of the vehicle, regenerative braking serves to cause the electric motor to operate as a power generator to thereby convert the kinetic energy of the vehicle into electric energy for braking. Furthermore, the obtained electric energy is stored in the secondary battery and reused at the time of acceleration and the like. Accordingly, in the vehicle that runs only by using the conventional internal combustion engine, the regenerative braking allows the reuse of the energy which has been dissipated into the atmosphere as heat energy, with the result that the energy efficiency can be significantly improved.
The secondary battery requires sufficient margin for effectively storing the electric power in the secondary battery which is generated at the time of regenerative braking. Furthermore, in the hybrid vehicle of a type in which the power generator can be driven by the heat engine mounted on the vehicle to generate electric power for charging the secondary battery, the electric power stored in the secondary battery, that is, the amount of the stored electric power can be freely controlled. Accordingly, in such a hybrid vehicle, it is desirable to control the amount of the electric power stored in the secondary battery to be approximately intermediate (50-60%) between a fully charged state (100%) and a non-charged state (0%) such that the regenerative power can be received and, if required, the electric power can be immediately supplied to the power generator.
The secondary battery mounted on the electric vehicle is used in various usage environments. When used in the cold climate area, the secondary battery may be used in the environment at a temperature of −10° C. or lower, and, in some cases, −20° C. or lower. Furthermore, as for the case where the secondary battery is used under high temperature conditions or where the secondary battery is used to cause an increase in temperature of the secondary battery, the secondary battery may be used in the environment at a temperature of 40° C. or above. When the secondary battery is used in the above-described severe environments, the control in accordance with the characteristics of the secondary battery is required. In particular, there is a problem that, at a low temperature, since the rate of the chemical reaction within the secondary battery decreases, a large current flowing therethrough causes a decrease in voltage, which prevents the required voltage from being achieved. In addition, there is also a problem that the secondary battery becomes deteriorated at a high temperature.
Japanese Patent Laying-Open No. 2003-219510 discloses a charge/discharge control device for a secondary battery which is capable of performing an appropriate charge/discharge control depending on the environment in which the battery is used and the battery state. The charge/discharge control device for a secondary battery disclosed in the above-mentioned document includes a temperature detecting unit detecting the temperature of the secondary battery, and a charge/discharge power limiting unit, in the case where the detected temperature is not more than a prescribed temperature, controlling the charge/discharge power not to exceed a predetermined charge/discharge power upper limit value which varies in accordance with the temperature.
In general, the higher the temperature of the battery is, the more the deterioration of the battery is accelerated. When the temperature of the secondary battery is not less than a prescribed temperature, the charge/discharge power limiting unit determines its upper limit value such that the upper limit value of the charge/discharge power is decreased as the temperature of the secondary battery rises. Consequently, a further increase in the battery temperature can be prevented, which allows the battery deterioration to be suppressed.
However, according to the technique disclosed in Japanese Patent Laying-Open No. 2003-219510, the electric power which can be obtained from the secondary battery varies in accordance with the temperature of the secondary battery. It is preferable that the electric power can be obtained from the secondary battery without limitation by temperature because the performance of the secondary battery can be more effectively achieved. Japanese Patent Laying-Open No. 2003-219510, however, does not disclose such a method.